Apparatus and method for transmitting an anisochronic data stream on an isochronic transmission route

ABSTRACT

To transmit an anisochronic data stream on an isochronic transmission route with a plurality of channels over a transmission network, a number of channels is reserved by the transmitter before beginning the transmission, wherein the sum of the transmission capacities of the reserved channels just exceeds the bandwidth of the asynchronous data stream. During the course of transmission, the data of the anisochronic data stream are formed into packets; each packet is transmitted over the reserved channels. After a packet has been sent, unused portions of the reserved channels are filled with filler data that is discarded at the receiver, until sufficient data are again available at the transmitter to begin transmission of the next packet.

BACKGROUND OF THE INVENTION

The invention relates to an apparatus that allows an isochronictransmission route (i.e., a digital transmission route operated with agiven system clock pulse) to transmit anisochronic data (i.e., data witha frequency different than the operating frequency of the transmissionroute).

The transmission of data (e.g., audio data) having a particular assignedfrequency for further processing or reproduction can encounter theproblem that the data must be transmitted on a clocked transmissionroute whose operating frequency differs from this assigned frequency.One possibility of transmitting anisochronic data on such a route is toconvert the assigned frequency to obtain an intermediate data streamwith the operating frequency of the transmission route. The intermediatedata stream is transmitted over the transmission route, and at thereceive side converted again to the assigned frequency. For example, inthe case of a data stream that includes an audio signal whose assignedfrequency is the sampling frequency of the audio signal, this frequencycan be converted by converting the signal to an analog signal and thensampling and redigitizing the analog signal at the operating frequencyof the transmission route.

However, a problem with this technique is that it is relativelycomplicated and expensive because of the rate converters needed at bothends of the transmission route. In addition, performing adigital-to-analog conversion and an analog-to-digital conversion causesa loss of quality in the audio signal and generally a loss of someinformation. If the operating frequency of the transmission route isless than the original sampling frequency, information loss isunavoidable even if the analogizing and digitizing processes are assumedto be ideally error-free, because high-frequency spectral components aresuppressed during transmission.

Therefore, there is a need for an apparatus that provides a transmissionsystem to transmit an anisochronic data stream, regardless of theoperating frequency of the transmission route and of the frequencyassigned to the data stream.

SUMMARY OF THE INVENTION

Briefly, according to an aspect of the invention, a data transmissionsystem includes a data bus and a data source that provides ananisochronic data stream. A transmitter receives the anisochronic datastream, and assigns a plurality of channels associated with the data busfor transmission of data indicative of the anisochronic data stream. Thetransmitter also partitions the anisochronic data into a plurality ofpackets and fills unused bit locations of the plurality of packets withfiller data, and provides output packets indicative thereof. A first businterface receives the output packets and transmits the output packetsonto the data bus. A second bus interface receives the output packets onthe data bus, and provides input packets indicative thereof. A receiverreceives and processes the input packets to recover the anisochronicdata stream, and provides a recovered anisochronic data streamindicative thereof.

Before transmission begins a number of channels of the transmissionroute are reserved, such that the sum of the transmission capacities ofthe reserved channels just exceeds the bandwidth of the anisochronicdata stream. In a preferred embodiment, the number of channels reservedis those needed to have available just more transmission capacity thancorresponds to the bandwidth of the anisochronic data stream. In thecourse of transmission, packets are formed from the data of theanisochronic data stream. Each of these packets is transmitted byutilizing the reserved channels. After a packet has been transmitted,the channels are filled up with filler data until enough data have againbeen collected at the transmitter to send another packet withoutinterruption. The filler data are discarded at the receiver, toreestablish the original anisochronic data stream.

To facilitate distinguishing useful data from filler data in the datastream received from the transmission route, a synchronization patternis preferably transmitted when transmission of each packet begins. Thesynchronization pattern signals to the receiver that the subsequent datais useful data.

The amount of useful data transmitted in each packet is preferablyfixed. Consequently, after receiving this fixed quantity of data, thereceiver can interrupt reception and stop processing the data streamfrom the transmission route, until a synchronization pattern againsignals a new transmission of useful data.

These features permit arbitrary, even non-commensurable, ratios betweenthe frequency assigned to the data stream and the operating frequency ofthe transmission route. Such frequency ratios can indeed have the resultthat the quantity of filler data transmitted between two successiveuseful data blocks varies. However, since the filler data is ignored bythe receiver, this does not cause the reestablishment/recovery of theoriginal anisochronic data stream to be more complicated on thereceiving end.

The channels of the transmission route are preferably time multiplexchannels. The transmitter thus has available cyclically changing timeslices of the various channels to transmit the data. The transmitterdecomposes the data words of a packet obtained from an external sourceinto units with the width of the channels, and transmits the resultingunits sequentially in free time slices of a reserved channel.

The present invention is especially suitable for transmitting streams ofaudio data. It can be used for example for a single audio channel, theleft or the right channel of a stereo signal, etc. In particular, in thecase of a stereo signal or other audio signals comprising severalchannels, it can be more economical for the transmission to form theaudio stream by nesting the data of several audio channels.

A MOST network is preferably used as the transmission route. Such anetwork preferably operates at a frequency of 44.1 MHz, corresponding tothe sampling frequency of a conventional CD. A DVD player, by contrast,yields a data stream whose individual output channels each have asampling rate of 48 kHz. In one embodiment, the method is especiallysuited for transmitting the anisochronic data stream delivered by a DVDplayer on MOST network. The anisochronic data stream can be a singleaudio channel of the DVD player with a sampling frequency of 48 MHz, orit can be composed of several audio channels of the DVD player. In thelatter case, a generalized sampling frequency, which is an integermultiple of 48 MHz, will correspond to this data stream.

These and other objects, features and advantages of the presentinvention will become more apparent in light of the following detaileddescription of preferred embodiments thereof, as illustrated in theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a block diagram illustration of a MOST network with a sourceand sink for anisochronic data connected to it; and

FIGS. 2 and 3 are pictorial illustrations of the transmission of theanisochronic data on the MOST network.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 is a block diagram illustration of a MOST network 100 with asource 3 and sink 5 for anisochronic data connected to it. The MOSTnetwork 100 includes a plurality of MOST interfaces 1, which areconnected to one another in a ring structure. A transmitter 2 isconnected to one of the interfaces 1. The transmitter 2 receives data,for example, having a word width of sixteen bits and a word frequency of48 kHz from the source 3 (e.g., a DVD player). The transmission ratefrom the DVD player 3 to the transmitter 2 thus is 768 kb/s.

The MOST network 100 between the interfaces 1 transmits data with a wordwidth of eight bits. The transmission time on the network is dividedinto frames with a repeat frequency of 44.1 kHz, where each frameincludes N time slices. In this embodiment, each of the N time slices an8-bit-wide data word which can be transmitted. The cyclically repeatingN time slices form N channels with a transmission capacity of 352.8 kHz(8 bits×44.1 kHz).

A receiver 4 receives from its associated interface 1 data circulated onthe MOST network 100 by the transmitter 2. The receiver 4 reconverts thedata stream into one with the original format delivered by the source 3(e.g., DVD player 3) to the transmitter 2, and forwards the converteddata stream to the sink 5 (e.g., an amplifier with a connectedloudspeaker). The mode of operation of the transmitter 2 and of thereceiver 4 will now be described with reference to FIGS. 2 and 3.

To transmit data from a DVD player over a MOST bus, three channels mustbe reserved for the data from the DVD player. Specifically, since theDVD player provides data at 768 kb/s, and each channel has a bandwidthof 352.8 kb/s, then three channels provide a bandwidth of 1.0584 Mb/s(3×352.8 kb/s), which is sufficient to transmit the data from the DVDplayer.

Before the transmitter 2 begins to output data to the interface 1assigned to it, the transmitter 2 first collects data received from theplayer 3 in a FIFO intermediate memory 6 until M data words have beencollected there. The number M of data words depends on the size of thepackets in which the data subsequently will be transmitted on the MOSTnetwork 100. As soon as the required data quantity is present in theintermediate memory 6, the transmitter 2 begins to send asynchronization pattern Sy 101 as illustrated in FIG. 2, on the channelsn₁ 102, n₂ 104, n₃ 106 assigned to it. When the receiver 4 receives thesynchronization pattern Sy, it recognizes that the followingtransmission will be a packet of M data words, which must be processedand forwarded to the sink 5.

Following the synchronization pattern Sy beginning with the time slicen₂ 108 of frame R2 110, the transmitter 2 begins to transmit the contentof the intermediate memory 6, here first with the most significant byteH1 112 and subsequently, in the time slice n₃, with the leastsignificant byte L1 114 of the first 16-bit-wide data word stored in theintermediate memory 6. In frame R3 116, bytes H2 118 and L2 120respectively of a second data word and byte H3 122 of a third data wordare transmitted.

While the transmitter 2 is processing the content of the intermediatememory 6 in this manner, the source 3 re-supplies data that is enteredin the intermediate memory 6, before they are transmitted. Since thetransmission data rate of 1.0584 Mb/s is greater than the rate at whichthe source 3 re-supplies data, the number of data words contained in theintermediate memory 6 declines in the course of transmission. However,the number M data words depends on the data rates of the player 3, ofthe transmitter 2, and of the packet size M. The data rate is chosen sothat the intermediate memory does not become empty before a completepacket of M data words has not been transmitted on the MOST network. Assoon as this number M of data words has been reached, the transmitter 2outputs filler data (e.g., bytes with the value 0) to the MOST network,until the intermediate memory 6 again contains M data words, which is asufficient data supply for transmitting another data packet withoutinterruption.

After the receiver 4 has received the synchronization pattern, itoutputs to the sink 5 the subsequently received M data words,reformatted into data words of 16-bit-width with a frequency of 48 kHz(i.e., in the same format as they were delivered from the player 3).After the sink has received the M data words, it ignores the datasubsequently transmitted on the channels reserved for the transmission,until it receives the next synchronization pattern.

In the example where the source 3 is a DVD player, the ratio of the datarates of the source 3 and the transmitter 2 is such that filler data aretransmitted on the network for about one-quarter of the time. However,in another embodiment, a more efficient utilization of the transmissioncapacity can be achieved if the anisochronic data stream delivered bythe source 3 is composed of several audio channels, each of whichincludes 16-bit-wide data words with a sampling frequency of 48 kHz. Asan example, transmission of a stereo signal is considered in FIG. 3. Inthis case, the total data rate of the source 3 is 1536 kb/s (2×768kb/s). To transmit this anisochronic data stream indicative of thestereo signal on the MOST network 100, five channels must be reserved,corresponding to a transmission rate of 1674 kb/s.

After a supply of M data words has been collected in the buffer 6 (wherethe numerical value of M here is different than in the case of FIG. 2),the transmitter 2 (FIG. 1) begins to transmit useful data in time slicen₂ 300 of the frame R′2 302. In the time slices n₂ 300, n₃ 304, thetransmitter 2 always sends the most significant and least significantbyte H1L, L1L of the first data word of the left channel of the stereosignal. Also, in the time slices n₄ 306, n₅ 308, the transmitter 2transmits the corresponding bytes H1R, L1R of the first data word of theright channel. In the subsequent frame R′3, the second data words of thetwo channels are transmitted and, in time slice n₅, the most significantbyte H3L of the left word of the left channel is transmitted. As alreadydescribed with reference to FIG. 2, the transmitter 2 continues totransmit until the given number M of data words of a packet has beenreached, and following this the transmitter 2 transmits filler datauntil the intermediate memory 6 again has reached the required datacomplement of M data words. As soon as this is the case, the transmitter2 transmits a new synchronization pattern, and the cycle repeats. Withthis variant, the fraction of time during which filler data aretransmitted on the MOST network 100 is less than 10%. As a result, thetransmission capacity of the network is used more efficiently.

The present invention has been described in one embodiment by way ofexample as a DVD player operating as the data source 3, and a MOSTnetwork 100 as a transmission route. However, one of ordinary skill willof course recognize that present invention also applies to data sourcesother than a DVD player, and synchronous transmission routes other thana MOST bus.

Although the present invention has been shown and described with respectto several preferred embodiments thereof, various changes, omissions andadditions to the form and detail thereof, may be made therein, withoutdeparting from the spirit and scope of the invention.

1. A method of transmitting an anisochronic data stream from a datasource to a data sink over an isochronic transmission network havingwith a plurality of channels, comprising: receiving data from the datasource, and reserving at least two of the plurality of channels toprovide reserved channels for transmission of data from the transmitteronto the transmission network, wherein the cumulative transmissioncapacities of the reserved channels exceeds the bandwidth of theanisochronic data stream; partitioning data of the anisochronic datastream into packets; filling bit locations of the packets not requiredto transmit the anischronic data with filler data, and providingpacketized data indicative thereof; and providing the packetized datafor transmission over at least one reserved channel of the transmissionnetwork.
 2. The method of claim 1, wherein said step of providing thepacketized data comprises inserting a synchronization pattern into saidpacketized data before data associated with the anisochronic data, toidentify the portions of the data of said packetized data as dataindicative of said anischronic data.
 3. The method of claim 1, whereinsaid packets each contain the same data quantity.
 4. The method of claim1, wherein said reserved channels are time multiplexed channels, and thetransmission network includes a time division multiplexed bus.
 5. Themethod of claim 1, wherein the anisochronic data stream comprises audiodata.
 6. The method of claim 1, wherein then transmission networkcomprises a MOST network.
 7. The method of claim 6, wherein the MOSTnetwork operates at a frequency of 44.1 MHz, and the anisochronic datastream has a frequency of 48 MHz or an integer multiple thereof.
 8. Themethod of claim 1, wherein the data source comprises a DVD player. 9.The method of claim 1, wherein the data source comprises a CD player.10. A data transmission system, comprising: a data bus; a data sourcethat provides an anisochronic data stream; a transmitter that receivessaid anischronic data stream, assigns a plurality of channels associatedwith said data bus for transmission of data indicative of saidanischronic data stream, partitions said anischronic data into aplurality of packets and fills unused bit locations of each packet withfiller data, and provides output packets indicative thereof; a first businterface that receives said output packets and transmits said outputpackets onto said data bus; a second bus interface that receives theoutput packets transmitted onto the data bus, and provides input packetsindicative thereof; and a receiver that receives and processes saidinput packets to recover said anischronic data stream, and provides arecovered anischronic data stream indicative thereof.
 11. The datatransmission system of claim 10, wherein said data bus includes a MOSTbus.
 12. The data transmission system of claim 11, wherein said datasource includes a DVD player.
 13. The data transmission system of claim10, wherein said MOST bus operates at a frequency of 44.1 MHz, and saidanisochronic data stream has a frequency of 48 MHz or an integermultiple thereof.
 14. The data transmission system of claim 13, whereinsaid reserved channels comprise time multiplexed channels, and said databus is configured and arranged as a time division multiplexed bus. 15.The data transmission system of claim 10, further comprising anintermediate memory device wherein said transmitter stores dataindicative of said anischronic data stream, and when a certain amount ofdata associated with said anischronic data stream has been stored insaid intermediate memory, said transmitter initiates providing saidoutput packets.
 16. The data transmission system of claim 15, whereinsaid transmitter also provides to said first bus interface asynchronization pattern that is transmitted over said data bus prior toeach of said packets associated with said anischronic data stream toidentify to said receiver said data associated with said anischronicdata stream.
 17. An apparatus for transmitting an anisochronic datastream from a data source to a data sink over an isochronic transmissionnetwork having a plurality of channels, comprising: means for receivingdata from the data source, and for reserving at least two of theplurality of channels to provide reserved channels for transmission ofdata from the transmitter onto the transmission network, wherein thecumulative transmission capacities of the reserved channels exceeds thebandwidth of the anisochronic data stream; means for partitioning dataof the anisochronic data stream into packets; means for filling bitlocations of the packets not required to transmit the anischronic datawith filler data, and for providing packetized data indicative thereof;and means for providing the packetized data for transmission over atleast one reserved channel of the transmission network.